Improve coordinator election logging in Iceberg Kafka Connect Sink

### Feature Request / Improvement

# Improve coordinator election logging in Iceberg Kafka Connect Sink

## Problem Description
The Iceberg Kafka Connect Sink connector fails silently when there's a mismatch between:
- The Kafka Connect consumer group ID (`consumer.group.id`)
- The Iceberg connector control topic group ID (`iceberg.connect.group-id`)

When this mismatch occurs, no coordinator is elected and no commits happen, but there are no clear error messages to help users identify the issue.

## Impact
When group IDs don't match:
1. No coordinator is elected
2. No START_COMMIT events are sent
3. Data is consumed but not committed to Iceberg tables
4. Users have limited visibility into why coordination isn't working

## Steps to Reproduce
The issue can be observed in these scenarios:

### Scenario 1: Explicit Different Group IDs
Configure Kafka Connect sink connector with:
```properties
consumer.group.id=connect-sink-group
iceberg.connect.group-id=different-group-name
```

### Scenario 2: Using Default Group ID (More Common)
Configure Kafka Connect sink connector with only:
```properties
consumer.group.id=connect-sink-group
# iceberg.connect.group-id not set, will use default value 'connect-iceberg-sink'
```

Key code references:
```java
// In CommitterImpl.java - Coordinator election process
private boolean hasLeaderPartition(Collection currentAssignedPartitions) {
ConsumerGroupDescription groupDesc;
try (Admin admin = clientFactory.createAdmin()) {
groupDesc = KafkaUtils.consumerGroupDescription(config.connectGroupId(), admin);
}
...

// In IcebergSinkConfig.java - Default group ID configuration
public static final String CONNECT_GROUP_ID = "iceberg.connect.group-id";
public static final String CONNECT_GROUP_ID_DEFAULT = "connect-iceberg-sink";
```

In both scenarios:
1. Start the connector
2. Observe that:
- Data is being consumed
- No commits are happening
- No clear error messages about group ID mismatch

## Root Cause
The `hasLeaderPartition` method in `CommitterImpl` checks for the control topic's consumer group existence but doesn't validate if it matches the actual Kafka Connect consumer group ID. This leads to a situation where:
1. The connector looks for members in the wrong consumer group (either explicitly configured or default `connect-iceberg-sink`)
2. No coordinator is elected because the group appears empty
3. The actual consumer group (where data is being consumed) is different

## Proposed Solution
1. Add appropriate logging when consumer group is not found
2. Provide clear messages to help users identify potential group ID mismatches

## Compatibility
This change is backwards compatible as it only improves logging without changing any behavior.

### Query engine

None

### Willingness to contribute

- [x] I can contribute this improvement/feature independently
- [x] I would be willing to contribute this improvement/feature with guidance from the Iceberg community
- [ ] I cannot contribute this improvement/feature at this time

Start in CommitterImpl at hasLeaderPartition and trace how KafkaUtils.consumerGroupDescription uses config.connectGroupId(). Read IcebergSinkConfig.java for CONNECT_GROUP_ID and its default, then inspect nearby coordinator-election logging. Done means users receive a clear message when the expected consumer group is absent or mismatched, with no behavior change.

Written by the indexing model from the issue text.